How about when the fluids were not on a book, but on someone's clothing?

When I worked for an academic library in Toledo, Ohio, we once had a series of complaints by female students, on Tuesday afternoons, always from the lesser populated corners of the 4th or 5th floors...

It always happened when the young female students were sitting on the floor, rummaging through books on the bottom shelf, when a man wearing a rain coat was noticed be VERY nearby.

Lets just say that the man we & the police were looking for was very silent when he reached the pinnacle of his activity -- while standing over the women who had no idea what had happened, until a thick fluid gradually moved down along their clothing.

The quiet perpetrator was caught by police on the 3rd week, while wearing the same telltale black raincoat.